Combustion in combustion chambers of gas turbine engines and in other industrial appliances needs to be performed over a large load range and at the same time with a minimum of pollutant emissions.
In gas turbine engines, frequently lean premix burners are applied, as for instance are known from, while not limited to those disclosed in, EP 321 809, EP 780 629, WO93/17279 or EP 945 677. However, individual burners of said type have only a quite limited range of loads, of burner equivalence ratios, respectively, in which they may be safely operated. With lower equivalence ratios at low load operation thermoacoustic oscillations may appear, or the flame may even completely extinguish. These lean premix type burners thus yield a very limited turndown ratio for part load operation. One possible approach to this extent may be the application of so-called pitot stages in the burners. The pilot stages are operated with diffusion combustion, having, due to the stoichiometry gradients inherent in diffusion combustion, a superior flame stability even at very low overall equivalence ratios. On the other hand, in diffusion combustion a zone of stoichiometric combustion is generally present, resulting in high peak temperatures and accordingly a high generation of thermally induced nitric oxides.
It is also known to apply so-called Constant Pressure Sequential Combustion CPSC. CPSC applies a front combustion stage at an understoichiometric equivalence ratio. Further fuel is injected into and combusted in the generated flow of still oxygen-rich combustion products. Generally, the front combustion stage requires the addition of mixer air in order to reduce the thermally induced formation of nitric oxides. The addition of mixer air, however, adds cost and bears the potential of inducing thermoacoustic oscillations.